Changing farming methods have led to widespread erosion. A plague is infesting the trees. Nonnative plant and animal species are wiping out indigenous flora and fauna.

The biggest threat by far, however, is the exploding human population, and what Mexicans refer to as the "mancha urbana," or urban stain.

"The statistics are clear," says Juan González Romero, who heads the district government. "In the early 1980s we were fewer than 180,000 inhabitants in Xochimilco. Now we have around 360,000 residents  about a 100 percent increase in only 20 years."

Illegal development in Xochimilco, which was named a World Heritage Site by UNESCO 15 years ago, has destroyed hundreds of hectares of protected land.

Low-income housing developments blot lands that used to be chinampas, or floating gardens. Cars and trucks rumble down pollution-choked streets that used to be canals.

Many of the development projects were built as a bid to win votes when Xochimilco, like most of Mexico, was controlled by the notoriously corrupt Institutional Revolutionary Party, or PRI.

In one famous case, a former PRI official brought in tons of rubble from construction sites around Mexico City to fill in a 10-acre lake bed. The ex-official is now in jail, according to Mr. González, and the building project he planned has been abandoned. The lake bed he filled in will probably never be fully recovered.

City dwarfed Paris 500 years ago

Aztecs had built the splendid Tenochtitlán  which at the time was far larger than European cities like Madrid, Rome, and Paris  atop a vast network of chinampas that spanned much of the Valley of Mexico.

Firsthand accounts from arriving Spaniards gush over the colorful pyramids, the lush gardens, and the flotilla of silent boats. Nonetheless, conquistador Hernan Cortes sacked the city, ousted his Aztec adversary, Montezuma II (known as Moctezuma in Spanish), and drained the canals to build roads.

Five hundred years later, Xochimilco remains the last living testimony to Mexico's Aztec past. It's a popular weekend spot for families and tourists alike, who drift down canals in colorful barges, sipping cool drinks and listening to the floating mariachi bands who sing songs about the canals for handfuls of pesos.

But the race is on to launch a rescue plan for what's left of the protected zone.

"The biggest challenge will be to stop the growth and constant building," says González. "We have succeeded in slowing down development in recent years ... but to make a total change will require a profound and integrated plan."

It's a plan that won't come cheaply.

District leaders in Xochimilco are asking for more than 3 billion pesos (about $300 million) to clean up and protect the 1,250-acre reserve, located about 14 miles south of Mexico City's center.

Mexico's cash-strapped government can hardly cover the cost. Yet past cases of corruption have left international donor bodies wary about dedicating funds to Xochimilco. UNESCO officials, for example, say privately that they fear future cash injections could be used to line the pockets of current functionaries.

Holding back rampant housing

Still, a handful of projects are already under way to try to save the area.

In one zone, trucks are removing rubble that was dumped as landfill to make way for other housing projects. It's a costly and time-consuming process  and brings no guarantee the area will ever return to its earlier state.

Another project seeks to rebuild chinampas the way the Aztecs did  using reeds and mud from the canal bed to create rich, undulating gardens. Trees known as ahuejotes are planted around the edge of the garden, and their roots eventually lock the site in place.

Aztec farmers slathered the straw beds with rich canal mud, producing as many as five crops a year on the same tiny plot. Nowadays, many local farmers have given up the slow, back-breaking process of dredging canal mud with canvas shovels, preferring to use fertilizers, which eventually exhaust the soil.

"Most people don't bother to use the mud anymore. They just put down chemicals," says farmer Anastasio Santana Delasco as he spreads wet earth on his crops. "I don't agree. You can't count on chemicals. With mud, the land always produces."

Deforestation has led to erosion

To make matters worse, locals also have given up caring for the ahuejote trees, leading to deforestation and rampant erosion that's crumbling chinampas across the protected zone.

"In the past, the people who lived and farmed here used to care for the trees themselves," says Sebastian Flores Farfan, a historian and scientist. "Not anymore. Now it's the district government that has to do it."

Biologist Uriel González Monzon is worried about another recent development: the spread of a parasite plant that's killing the ahuejotes. The parasite inserts itself on the branch of a normal tree and grows off a tumor-like bulb. Its roots feed off the tree, sprouting huge branches that eventually starve the ahuejote.

Dr. González is overseeing a complicated process to cut infected branches off hundreds of thousands of trees that line the chinampas. "There are some trees which have to be cut back almost to the trunk for the gravity of the infestation," he explains at one site where workers used machetes to hack off infected branches. "But this tree has the advantage of being able to grow back quickly. It has a great capacity for regenerating its branches."

Scientists and local officials alike say projects like these won't succeed without significant outside help. Other plans aim to rescue hundreds of rare plant and animal species, some of which exist only in Xochimilco. There are also programs to stop local residents from dumping sewage in the canals and to start projects to fight tourist litter.

"This region was named a World Heritage Site in 1987 precisely because it is the last example of the how the entire Valley of Mexico looked in the pre-Hispanic era," says district leader González. "This isn't just for our residents. Xochimilco is unique in all the world."